This invention relates to methods and devices that are used for the excitation, transmission and reception of ultrasonic waves for non-destructive testing of various materials that are usually in the solid state. Nondestructive ultrasonic tests (UT) use very short acoustic waves that are composed of center frequencies typically in the range of 0.1 to 15 MHz and occasionally up to 50 MHz. Ultrasonic transducers convert electrical pulses into mechanical vibrations which are transmitted as beams of ultrasonic waves that propagate into said materials. Said ultrasonic waves reflect back to the transmitting transducer or are transmitted to other receiving transducers to produce electrical signals. Said signals are used to detect internal defects or to measure the chemical or physical properties of said materials.
UT is often performed on steel and other metals and alloys, although it can also be used on concrete, wood and composites, with some loss of sensitivity and resolution. UT is a method of nondestructive testing used in many industries including aerospace, automotive and transportation infrastructure such as pipelines and bridges. UT is frequently used to determine the thickness of the test component material, and therefore can be used to monitor defects such as corrosion in pipe. Also, it can be used to measure the metallurgical properties such as the nodularity of cast iron and physical properties such as the stress load on bolts.
Conventional UT is performed by using a transducer that is composed of a piezoelectric material to generate a compression wave, also known as a longitudinal (L) wave, as illustrated in FIG. 1. The L wave is generated by the abrupt application of a differential electrical potential to plates (1) and (2) through electrical conductors (3) and (4). When a positive voltage is applied to plate (1) a negative voltage or positive voltage of lesser magnitude is applied simultaneously to plate (2). This difference in electrical potential produces an electrical field that permeates the piezoelectric material (5) installed between the plates with the provision that said material is a poor electrical conductor, e.g. insulator. If said material exhibits piezoelectric properties, it will expand in the direction of its thickness by an amount that is proportional to the difference in the voltages applied to said plates. The thickness of said material decreases when the said electrical potentials are removed.
Application of a short electrical voltage pulse between plates (1) and (2) causes a rapid expansion and contraction of said material between said plates. Said rapid expansion and contraction of said material produces L waves that radiate acoustic energy in two directions, each being perpendicular to the plane of said plates. Said L waves (6) that radiate from the bottom plate (2) propagate through a coupling media (7) such as water, oil or rubber and into the test component (8) i.e. structure or part of a mechanism that is being interrogated. Reflections of said L wave from physical features or defects of said test component return to the transducer where they induce electrical signal responses by the reverse piezoelectric process, i.e., compression of piezoelectric material that produces electrical charges of opposite polarity on plates on the sides of said piezoelectric material. The electrical charges on said plates cause said electrical signals that can be observed at said electrical terminals connected to said plates. Said L waves that radiate from the top plate (1) are attenuated by damping material (9) to prevent penetration into the support backing material (10) and housing (11). Installation of said damping material eliminates the possibility of false indications and erroneous measurements caused by ultrasonic reflections within said backing material and housing.
Some UT applications require the use of shear waves instead of or in addition to said L waves. Although electric field transducers can be used to produce vertically polarized shear waves, the primary method of generating said shear waves results in a beam of energy that radiates at an angle that is less than 90 degrees with respect to the surface of the test material rather than a normal beam that is perpendicular to said surface. An alternate method of generating shear waves from piezoelectric transducers requires the use of viscous couplant such as honey or glue that has at least partial adhesion to the test component surface. The use of said viscous couplants for shear wave inspections is often a slow, tedious and costly process. Since many UT applications require the use ultrasonic waves that radiate from the transducers as normal beams, it is a valid conclusion that the use of piezoelectric transducers to generate shear waves for these applications is impractical for many applications.
An EMAT is the ideal transducer for many UT applications that require normal beam shear waves. FIG. 2 illustrates an EMAT composed of a circular spiral coil of insulated conductor such as copper wire (12) and a permanent magnet (13) which produces a relatively strong magnetic field, e.g., 5 kilogauss, in the test component and has a predominant component vector that is perpendicular to the test component surface. When an alternating current pulse is applied through lead wires (14) and (15) to said spiral coil an alternating magnetic field that encircles the windings of the coil is induced in the test component (8) in the immediate vicinity of said coil. Part of said induced magnetic field penetrates said test material and induces eddy currents (16) and (17) that flow in a solenoidal path and in a direction that is opposite to the current in said coil. When current flows into the outside lead (15) of said coil the eddy currents will flow in a clockwise direction, looking down on said spiral coil. This is indicated by eddy current direction (16) out of the page and eddy current direction (17) into the page of FIG. 2.
Interaction of said magnetic field from said permanent magnet with said eddy currents produces alternating, horizontal, shear forces that move the lattice of said test material either away from or toward the center of said coil. Said alternating shear forces generate horizontal-polarized shear (SH) waves [19] near the surface of the test material that propagate in direction that is normal or perpendicular to said test material surface. To prevent the induction of said shear waves in said permanent magnet, a ferromagnetic material that also has relatively high electrical resistivity (18) is installed between said coil and the pole of said permanent that is next to said coil. The magnetic field is concentrated in said ferromagnetic material and therefore diminished in the pole of the magnet so that relatively weak eddy currents and resulting weak shear waves are generated in said magnet. Also, said ferromagnetic material serves as a shield from transient magnetic fields caused by reflections within said magnet. Said shielding can be enhanced by attaching a thin sheet of electrical conductor such as copper to the magnet pole.
Two basic methods of receiving the ultrasonic waves, pulse-echo method and through-transmission method, are frequently used for nondestructive testing. The pulse-echo method uses the transducer to transmit and receive said ultrasonic waves. Said pulse-echo method pertains to reflected ultrasonic wave that comes from an interface, such as the back wall of the object or an imperfection within the object that returns to said transducer. Said ultrasonic wave is recorded as a signal having an amplitude that is proportional to the intensity of the reflected wave and with a time of arrival that is proportional to the travel distance of said reflection. In the attenuation or through-transmission method, a transmitter sends an ultrasonic wave through one surface, and a separate receiver on another surface detects and processes said ultrasonic wave that has reached said receiver after traveling through said test component. Imperfections or other conditions in the volume of test component material, which are located between the transmitter and receiver, change the characteristics of said ultrasonic wave, thus revealing their presence, location and in some cases physical characteristics within said component material.